[swift-evolution] Handling unknown cases in enums [RE: SE-0192]

Jose Cheyo Jimenez cheyo at masters3d.com
Wed Jan 10 14:03:52 CST 2018

On Tue, Jan 9, 2018 at 10:10 PM, Chris Lattner <clattner at nondot.org> wrote:

> On Jan 9, 2018, at 3:07 PM, Jose Cheyo Jimenez <cheyo at masters3d.com>
> wrote:
> > Hi Chris,
> >
> > This is great. Thanks for spending time on this! I am in favor of `case
> #unknown` to only match unknown cases.
> >
> > 1) Would #uknown be available to RawRepresentable structs?
> I haven’t considered this, so I’m not sure how that would work.  I had
> assumed that this would be an enum specific concept.
> What do you have in mind?  If your raw representation is itself an enum,
> then of course this would work.

I was just curious since a RawRepresentable structs ( and OptionSet ) are
very similar to non exhaustive enums in that they would require handling
unknown cases. Currently RawRepresentable is required to use a default even
if all cases are accounted for.

struct Directions: OptionSet {
    let rawValue: UInt8
    static let up    = Directions(rawValue: 1 << 0)

let myDir = Directions.up

switch myDir
    case .up : print("matched")
    default: print("default required")

> > 2) How is the #uknown pattern accomplished? Are you suggesting to
> capture all the compile time known cases so you can do a diff during
> > runtime? (Sorry this is not obvious to me)
> The internals of the compiler would handle this, it is difficult to
> explain unless you know the bowels of silgen and irgen :-)
> -Chris
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